Stress/strain and other material strength characteristics of an elongated sample of material are measured in a load frame by engaging a sample at its two ends by grips which apply compressive or tensile loads or both along the longitudinal axis of the sample (axial loading).
It is difficult to get the sample perfectly aligned with the two grips, resulting in uneven loading of the sample (e.g., a 5 to 7% difference in stress at different locations across a cross-sectional slice of a sample). Uneven loading in metal samples causes them to bend, tending to redistribute the stress over a larger area and to reduce the effect on test results of uneven loading. Ceramics, however, are brittle and may crack before desired test loads have been applied.
The typical testing of ceramics involves using a "bend test" in which an elongated, rectangular cross-section sample is supported from below at its ends, and a downward force is applied to the middle, placing the top in compression and the bottom in tension. A limitation of this test for ceramics relates to the cause of failure of ceramics, namely a crack initiated at one of a plurality of flaws within the material when that flaw is subjected to sufficiently high stress. In the bend test only the surface region across the middle of the bottom of the sample is subjected to the highest tensile stress, and there is a smaller probability that a critically sized flaw will be present there. The bend tests can thus give ultimate stress values that are higher than they should be, making them unreliable. With axial loading, on the other hand, the maximum stress is applied uniformly (subject to what was said above about uneven loading) across the width of the entire sample and over its entire length, making it virtually certain that a flaw, wherever located, will be subjected to the maximum stress.